HOLD ON HOLD ON.............
mklyotskin, Ignore all of the above. In the next service pack of Enterprise Vault there will be a feature called "Move Archive" this will move an entire archive from one place to another.
So you can move all your old archives to a new Vault Store Group that has OSIS enabled. We only speaking a couple of months here, so I would go ahead and plan your upgrade to v8.0 SP2, and when the next service pack hits the street you are ready to rock and roll.
I sat on most of the Move Archive reviews, and yes it will do everything you need it to in your case.
--wayne